The invention relates to a process for the preparation of compounds having at least one xe2x80x94CF2Oxe2x80x94 bridge in the molecule.
Liquid crystals have been widely used ever since the first commercially useful liquid-crystalline compounds were discovered about 30 years ago. Typical application areas are in particular displays for watches and clocks or pocket calculators, or large display panels as used in railway stations, airports or sports arenas. Other application areas are displays of portable computers or navigation systems, and video applications. The latter applications in particular have to meet high requirements on switching times and image contrast.
The liquid-crystalline molecules must have certain properties to be useful for commercial applications. To be able to use devices having a liquid-crystal display under various climatic conditions, the molecules must form a stable nematic phase over a very wide temperature range including the room temperature range. The compounds must therefore have a low melting point and a high clearing point.
To be able to realize low switching times, the molecules must have a low rotational viscosity. For example, switching times of less than 16.7 ms are required for video applications. The liquid-crystalline molecules should furthermore have a high dielectric anisotropy so that only low threshold voltages are required. This means that little energy is required so that it is possible to use smaller and lighter accumulators, for example in laptops. Another important factor for the design of the display are the birefringence properties of the molecules which have an effect on contrast and available viewing angle.
To be able to satisfy all these requirements simultaneously, mixtures which often comprise 5 to 15 different components are employed rather than pure substances. This means that the individual components have to be compatible with one another, i.e. are sufficiently soluble in one another, for example.
High image contrast is desired for modern active-matrix displays. The liquid-crystalline compounds must therefore have a high resistivity and a high voltage holding ratio.
Liquid-crystalline compounds which have been found to have a particularly high resistivity are those which contain fluorine-containing groups in their molecular framework. For example, EP 0 844 229 A1 describes liquid-crystalline compounds which contain an xe2x80x94Oxe2x80x94CF2xe2x80x94 bridge. Various methods for obtaining this xe2x80x94Oxe2x80x94CF2xe2x80x94 bridge are suggested. One of the methods described involves initially converting an aromatic halide into a Grignard compound or a lithiated compound followed by conversion into the the dithiocarboxylic acid by means of carbon disulfide. The dithiocarboxylic acid is then converted into a thioester using a phenol in the presence of an alkali metal hydride and iodine. The desired xe2x80x94Oxe2x80x94CF2xe2x80x94 bridge is then formed from the thioester using a fluorinating agent.
Another method which is suggested involves initially reacting a cyclohexanone with tris(dimethylamino) phosphine and dibromodifluoromethane to obtain a difluoromethylenehexylidene. A xe2x80x94CF2xe2x80x94Oxe2x80x94 bridge is then formed by addition of bromine to this derivative followed by etherification by reacting with a phenolate with simultaneous elimination of hydrogen bromide.
Disadvantages of these processes are low reaction rates, unsatisfactory yields and complicated work-up and purification.
It is therefore an object of the invention to provide a process for the preparation of compounds having at least one xe2x80x94CF2Oxe2x80x94 bridge in the molecule which produces good yields at a satisfactory reaction rate. Intermediates and final products should be easy to purify.
This object is achieved by the process according to the invention, which comprises initially reacting a bis(alkylthio)carbenium salt with a compound containing at least one hydroxyl group in the presence of a base followed by oxidative fluorination, preferably in situ, with a fluorinating agent and an oxidizing agent to form the compound having at least one xe2x80x94CF2Oxe2x80x94 bridge in the molecule.
The bis(alkylthio)carbenium salts can be prepared very easily from the corresponding carboxylic acids or activated carboxylic acid derivatives. Examples of suitable carboxylic acid derivatives include carbonyl halides, carbonyl pseudohalides, suitably substituted carbonyl sulfonylates, such as a trifluoromethylsulfonylate. Furthermore, carboxylic anhydrides and alkyl- or phenylcarboxylic esters can be used. The salts precipitate in clean form from the reaction solution and can be employed in the next stage without further purification.
The bis(alkylthio)carbenium salt is initially reacted with the compound containing at least one hydroxyl group to produce a dithioorthoester. This dithioorthoester is generally not isolated, but immediately reacted further. The oxidative fluorination to form the compound having a xe2x80x94CF2Oxe2x80x94 group is conducted under very mild, slightly basic conditions and is therefore compatible with numerous unprotected functional groups, for example a nitrile group, in contrast to the conventional methods. Another advantage is that the stereochemistry of the radicals, for example a cis-oder trans-cyclohexylene radical, is retained in the reaction. The principal steps are summarized in FIG. 1 below. 
FIG. 1 Reaction scheme for the preparation of compounds having a CF2O bridge.
The carboxylic acid derivative A, in which X is, for example, xe2x80x94OH, halogen, pseudohalogen, substituted sulfonate, an anhydride, alkoxy or phenoxy, is then reacted with an alkylthiol to give the bis(alkylthio)carbenium salt B. Preference is given to using dithiols which lead to the formation of a cyclic cation. Particularly suitable thiols are therefore ethanedithiol, propanedithiol or 1,2-benzenedithiol which lead to the formation of dithianylium and dithiolanylium salts, respectively. This salt B is then reacted with a hydroxyl compound Rbxe2x80x94OH to give an orthoester C. The orthoester C is generally not isolated, but directly oxidized to give compound D. The process is universally applicable so that Ra and Rb are not subject to any restrictions per se. For example, Ra and Rb, independently of one another, can be an alkyl, aryl, cycloalkyl or alkenyl radical, where these radicals in turn may be substituted as desired, for example by halogen, pseudohalogen, hydroxyl or carbonyl groups.
For the preparation of liquid crystals it is preferred that the bis(alkylthio)-carbenium salt has a structure of the formula I. 
in which: 
in each case independently of one another, is cis- or trans-1,4-cyclohexylene, 1,4-phenylene or 1,4-cyclohex-3-enylene, where these groups may also be monosubstituted or disubstituted by halogen, in particular fluorine, pseudohalogen, xe2x80x94OCF3 or xe2x80x94OCHF2,
R1 is a linear or branched alkyl or alkoxy radical having 1 to 12 carbon atoms which is unsubstituted, monosubstituted by xe2x80x94CN or xe2x80x94CF3 or at least monosubstituted by halogen, a linear or branched oxyalkyl, alkenyl or alkenyloxy radical having 2 to 12 carbon atoms or a linear or branched oxalkenyl radical having 3 to 12 carbon atoms, where, in addition, one or more CH2 groups in these radicals may, in each case independently of one another, be replaced by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94 or xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94 in such a way that heteroatoms are not linked directly to one another, or halogen, preferably F or Cl,
Z is, in each case independently of one another, a single bond, a xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CF2xe2x80x94CF2xe2x80x94, xe2x80x94CF2xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CF2xe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CFxe2x95x90CFxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CF2xe2x80x94Oxe2x80x94 or xe2x80x94Oxe2x80x94CF2xe2x80x94 group,
xe2x80x94Sxe2x80x94(CR2R3)nxe2x80x94Sxe2x80x94 is a bridge consisting of two sulfur atoms which are linked by a carbon chain having two or three carbon atoms, where the hydrocarbon bridge may also carry one or more substituents R2 and R3, in particular alkyl or alkylene groups, where, in addition, the radicals R2 and R3 may together form a cycloalkyl group or aryl group,
The process according to the invention is generally suitable for the preparation of compounds having a xe2x80x94CF2xe2x80x94Oxe2x80x94 bridge, such as liquid crystals, intermediates for polymers, pharmaceuticals and crop protection agents, for example. However, it is particularly suitable for the preparation of liquid-crystalline compounds. As a fragment which is adjacent to the xe2x80x94CF2xe2x80x94Oxe2x80x94 bridge on one side is introduced into the molecule by means of the bis(alkylthio)carbenium salt B, the radicals R1 are preferably structural elements customary in liquid-crystalline compounds. Some fragments of this type are listed by way of example in FIG. 2, this list not being exhaustive. R is as defined for R1 above. 
FIG. 2 Examples of building blocks customary in liquid-crystalline compounds.
The compound containing at least one hydroxyl group is preferably a phenol which may optionally be substituted, and in particular carries a polar group, preferably xe2x80x94F, xe2x80x94Cl, xe2x80x94CN, xe2x80x94NCS, xe2x80x94OCF3 or xe2x80x94OCH2F, or an alkyl, cycloalkyl or phenyl radical in 4-position, which radicals in turn may be substituted by alkyl, cycloalkyl or phenyl radicals, where, in addition, hydrogen atoms may also be substituted by fluorine or chlorine atoms and in each case a xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CF2xe2x80x94CF2xe2x80x94, xe2x80x94CF2xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CF2xe2x80x94, xe2x80x94CFxe2x95x90CFxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94CF2xe2x80x94 or xe2x80x94CF2xe2x80x94Oxe2x80x94 group may be present between these groups.
The structure of the compound containing at least one hydroxyl group is not subject to any particular restrictions per se. However, as the process according to the invention is particularly suitable for the preparation of liquid crystals, the radical present in the compound containing at least one hydroxyl group preferably contains structural elements customary in liquid crystals. Examples of structural elements which may be mentioned are again the building blocks depicted in FIG. 2.
The bis(alkylthio)carbenium salt preferably contains a noncoordinating or weakly coordinating anion, in particular selected from the group consisting of tetrafluoroborate, hexafluorophosphate, perchlorate and perfluoroalkylsulfonate, in particular trifluoromethanesulfonate. Owing to their very low hygroscopicity, these salts are easy to process.
The oxidizing agent used can be any customary oxidizing agent. The oxidizing agent used is preferably a compound which releases halonium equivalents. Examples of oxidizing agents are N-bromosuccinimide, N-iodosuccinimide, 1,3-dibromo-5,5-dimethylhydanthoin and bromine. Particular preference is given to bromine, since the resulting bromides are easy to remove. Other examples of suitable oxidizing agents are SO2Cl2, SO2ClF, nitrosonium and nitronium salts and Chloramin T.
The fluorinating agent used can be any customary fluorinating agent. The fluorinating agent is particular preferably selected from the group consisting of aliphatic and aromatic amine/hydrogen fluoride complexes, for example pyridine/hydrogen fluoride complexes, NEt3xc2x73HF, 50% HF in pyridine, melamine-HF and polyvinylpyridine-HF.
As already mentioned above, the process according to the invention is particularly suitable for the preparation of liquid crystals. The process according to the invention utilizes bis(alkylthio)carbenium salts as starting materials. The invention therefore also relates to bis(alkylthio)carbenium salts of the formula I. 
in which R1, 
Z, xe2x80x94Sxe2x80x94(CR2R3)nxe2x80x94Sxe2x80x94, m, n and Yxe2x88x92 are as defined above.
The invention furthermore relates to dithioorthoesters of the formula II 
in which R1, R2, R3, Z, 
m and n are as defined in claim 2, and R4 is as defined for R1 in claim 2, and, in addition, may be xe2x80x94H, xe2x80x94F, xe2x80x94Cl, xe2x80x94CN, xe2x80x94SCN, xe2x80x94OCF3 or xe2x80x94OCHF2, and o is an integer between 0 and 6.
The invention is illustrated below by means of examples.